Process of dimerizing carboxylic acids in a corona discharge



United States Patent 3,533,932 PROCESS OF DIMERIZING CARBOXYLIC ACIDS IN A CORONA DISCHARGE John A. Coifman, Ballston Spa, and William R. Browne, Scotia, N.Y., assignors to General Electric Company, a corporation of New York No Drawing. Original application Mar. 26, 1965, Ser. No. 443,159, now Patent No. 3,356,602, dated Dec. 5, 1967. Divided and this application Oct. 2, 1967, Ser. No. 671,989.

Int. Cl. B01k l/OO; C11k 1/00; C07c 55/07 US. Cl. 204-167 5 Claims ABSTRACT OF THE DISCLOSURE The invention relates to a process of treating with a corona discharge a carboxylic acid having 0 to 3 ethylenic linkages and from 2 to 30 carbon atoms or esters of such an acid and an alcohol having 1 to 6 carbon atoms in a reducing atmosphere, preferably hydrogen, at a pressure of at least one atmosphere. Monoand tri-esters are exemplified.

This invention is related to that disclosed and claimed in Ser. No. 443,159, filed Mar. 26, 1965 now US. Pat. 3,356,602 to the same inventors and common assignee and is a divisional application thereof.

The invention relates to a process of treating carboxylic acids and esters thereof within a corona discharge.

It has long been recognized that gaseous electrical discharges are capable of effecting chemical alterations. When two bare, spaced electrodes are subjected to a large potential difference within the atmosphere, the portion of the atmosphere lying between the electrodes is rapidly ionized so that an electrically conductive path is provided between the electrodes. The resulting high current, low voltage electrical discharge is an arc and is readily identified visually by its limited areal extent and sharply defined boundaries. An electric arc is chemically highly disruptive and for this reason unsuited for treating organic materials except where a high degree of fragmentation is desired.

The terms silent electric discharge and voltolization refer to high voltage, low current electrical discharge phenomena clearly distinguishable from an electric are by being soft and diffused. Reference to silent electric discharge or voltolization conducted at pressures substantially beneath atmospheric is indicative of glow discharge, which is a bare electrode phenomenon sustainable with alternating or direct current. The electrodes are maintained bare to produce contact ionization of the surrounding atmosphere. In order to prevent ionization from bridging the electrodes-i.e., generating an arc it is necessary that the atmospheric pressure be reduced. The pressure reduction greatly increases the mean-free path of chemically active ions and electrons between collisions thereby increasing the mean particle velocity. Such prop- 'erties are recognized, for example, in Reactions of Hyshorter mean-free path between particle collisions and reduced mean particle velocities. Accordingly, chemical treatments by corona discharge are substantially less disruptive in character than those obtainable by either elec tric are or glow discharge treatments.

It is an object of our invention to provide a process of treating carboxylic acids and esters in an electric discharge and a non-oxidative atmosphere to achieve a useful net change in the materials without decarboxylation, without introducing oxidative color impurities, and without complete elimination of any unsaturation which may be present. In the case of monoesters and unsaturated polyesters, it is an object to achieve a net dimerization of the material. It is a further object to treat carboxylic acids in a non-reactive atmosphere.

It is our discovery that useful products may be obtained by subjecting monocarboxylic acids and esters thereof to a corona discharge propagated in a reducing atmosphere maintained at or above atmospheric pressure. Whereas electrical discharge treatments of organic materials have heretofore been considered to yield more or less random chemical associations and cleavages, we have discovered that in corona treatment of monocarboxylic acids and esters certain types of reactions will predominate to produce useful net changes in the materials. When monocarboxylic acids and monoesters are treated in a corona discharge, a net dimerization is observed regardless of whether unsaturation is present or absent. In corona treatment of polyesters of monocarboxylic acids preponderant dimerization is observed when the acid moiety is predominately unsaturated, as in polyunsaturated tri-esters while molecular cleavage is predominant in relatively saturated tri-esters. It is our discovery that corona discharge dimerization may be sufficiently controlled to retain a substantial degree of original unsaturation. Our process accordingly appears well suited to place liquid poly-unsaturated oils in conveniently usable solid form without the impairment of unsaturation required by conventional processes. A notable advantage of our corona treatment process is that net changes can be effected in carboxylic acids and esters without decarboxylation. Further, by using a reducing gas atmosphere, low color products substantially devoid of discoloring impurities of generally oxidative origin can be obtained. The atmospheric composition is also of utility in modifying the net chemical change produced by corona treatment. Finally, we have discovered that corona treatment of a monocarboxylic acid or ester can be more efficiently achieved in a packed dielectric bed.

As previously noted, a corona discharge is a soft, diffused visual display, capacitative in nature, requiring for propagation at least one dielectric barrier between spaced electrodes connected to a source of alternating current. When dielectric barriers are employed adjacent each of two spaced electrodes in achieving a corona discharge, the discharge phenomenon is frequently termed an electrodeless discharge, whereas when a single dielectric barrier is employed to insulate a single electrode, the resulting phenomenon is frequently termed a semicorona .discharge. Both electrodeless and semicorona discharges are included within the scope of the invention and are included by the generic term corona discharge.

A corona discharge is a gas phase phenomenon. When a gas is placed in an electric field generated by spaced electrodes connected to a source of alternating current, the gas absorbs energy fromthe electric field. The energy asborption may result in activating electrons into higher energy orbitals, in dissociating diatomic gases into free racials, in forming gaseous ions, or in any combination of these. Further, since the electric field of an alternating current cyclically decays, the absorbed enery is cyclically liberated by the gases.

When it is desired to treat a normally liquid material such as a carboxylic acid or ester thereof, it is necessary to produce the corona discharge in an atmosphere in contact with at least one surface of the liquid to be treated. The exposed surface to be treated may be a static surface as, for example, the surface of a liquid confined in a beaker. More eificient treatments are achieved in dynamic systems where the material to be treated is flowed through the corona zone in the form of a curtain or film. We have discovered that highly efficient treatments may also be provided by flowing a liquid to be treated through a packed bed of dielectric particles. Efiicient treatments may ordinarily be obtained using packed beds having particles ranging from as low as 0.07 mm. in longest dimension. The packed bed serves to greatly multiply the surface area of the material in contact with the corona and to improve distribution of the corona. Any solidified material produced within the packed bed during corona treatment can be easily removed by contemporaneous or subsequent heating of the packed bed by conventional means.

It is our discovery that particular advantages may be achieved in treating a carboxylic acid or ester in a reducing gas atomsphere. By treating a carboxylic acid or ester in a reducing atmosphere, any tendency toward oxidative degradation of the organic molecule can be obviated. Hydrogen constitutes a preferred reducing atmosphere, since its relatively simple structure allows it to be easily activated to generate a corona discharge. Also, the hydrogen atmosphere is capable of interacting with the organically bound hydrogen atoms without generating elemental impurities.

It is a further procedural advantage of our invention to treat a carboxylic acid or ester with a corona discharge propagated in a gaseous medium maintained at or above atmospheric pressure. Inasmuch as corona discharge is maintained between spaced electrodes and arcing prevented by the dielectric properties of the barrier material rather than by the atmosphere density, as is the case in glow discharge, a corona discharge may be maintained at pressures up to atmospheres or higher. Operation at ambient or positive pressures allows elimination of expensive and cumbersome vacuum producing equipment. Additionally, the explosion hazard due to air leakage into a corona propagating reducing amosphere such as hydrogen, for example, is avoided.

A corona discharge can be generated using only an alternating curent. Generally, high frequencies are preferred since the phenomenon is capacitive in nature. Frequencies ranging from c.p.s. to 500,000 c.p.s. are contemplate. A preferred frequency range is from 3,000 c.p.s. to 10,000 c.p.s. The current, voltage, and power utilized in generating a corona discharge for a specific process will vary over wide limits depending on the thickness of the dielectric barrier or barriers employed, the electrode spacing, and the nature of the gaseous media lying Within the discharge area. It is generally preferred that the corona producing gap between spaced electrodes be no more than /2 inch. Preferred total barrier thicknesses range from A2 inch to A inch, irrespective of Whether one or two barriers are employed. In a hydrogen atmosphere, with a 4; inch quartz barrier. an exemplary desired voltage range extends from 8 to 16 kv. peak.

Materials to be treated with a corona discharge according to our invention include monocarboxylic acids, preferably those having from 2 to 30 carbon atoms and most preferably those having 10 to carbon atoms. The acids may be saturated or unsaturated. Preferred acids are those having up to three ethylenic linkages. Exemplary saturated carboxylic acids include butyric, isovaleric, caproic, caprylic, capric, lauric, myristic, palmitic, stearic arachidic, behenic, lignoceric, ceratic, etc. Exemplary unsaturated carboxylic acids include A -decylenic, stillingic, A -dodecylenic, palmitoleic, oleic, ricinoleic petroselinic, vaccenic, linoleic, linolenic, eleostearic, licanic, parinaric, gadoleic, arachidonic, cetroleic, erucic, selacholeic, hydnocarpic, chaulmoogric, gorlic, mycolipenic, mycoceranic,

etc., as well as stereoisomers thereof and other similar acids. If desired, acids having acetylenic as well as ethylenic unsaturation may be treated. Included thereby are such acids as ximenynic, erythrogenic, mycomycin, isomycomycin, etc., as well as stereoisomers and other similar acids. It is generally preferred to choose those acids for treatment having isolated ethylenic linkages, since less opportunity for energy absorption through resonance is afforded.

Our invention may additionally be practiced with esters of acids of the type above noted, preferably with those having alcohol moieties of less than six carbon atoms. The term alcohol as herein employed designates hydrocarbon derivatives in which one or more hydrogen atoms are replaced by hydroxyl groups. Suitable alcohols include primary, secondary, and tertiary monohydric alcohols as well as dihydric alcohols, trihydric alcohols, etc. Suitable esters include those capable of yielding upon hydrolysis such exemplary alcohols as methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, allyl alcohol, n-butyl alcohol, isobutyl alcohol, sec-butyl alcohol, t-butyl alcohol, n-amyl alcohol, isoamyl alcohol, t-amyl alcohol, neo-pentyl alcohol, ethylene glycol, propylene glycols, butylene glycols, glycerol, erythritol, pentaerythritol, etc. Esters of particular interest are fats and oils including, for example, coconut oil, ba bassu oil, palm kernel oil, palm oil, olive oil, castor oil, peanut oil, rape seed oil, lard, whale blubber, corn oil, safflower oil, cottonseed oil, soybean oil, triolein, trilinolein, trilinolenin, tristearin, tricaprin, trilaurin, trimyristin, tripalmitin, etc.

The materials to be treated on a corona discharge may be mixtures of carboxylic acids and/or esters thereof. The materials may be derived from natural or synthetic sources. The energy to be expended on the materials will depend on the particular material to be treated together with the net structural rearrangement desired. Generally, treatments in the range of from 0.01 to 10 watt-hours/ gram are effective to produce substantial net changes in the material, with treatments in the range of 0.1 to 5 watt-hours/ gram being preferred.

Carboxylic acids as well as unsaturated monoesters thereof subjected to corona discharge according to the present invention show visibly discernible viscosity increase. Liquids may be solidified by treatment. Tests of materials treated indicate net dimerization and absence of decarboxylation. Unsaturated materials retain unsaturation subsequent to treatment. In the case of substantially saturated fatty esters, the rate of molecular cleavage is believed to exceed the rate of dimerization so that a net loss of molecular weight is exhibited. It is believed, however, that both useful dimers and cleavage products may be obtained through conventional chemical separation techniques.

Our invention may be better understood by reference to the following examples which are intended to illustrate rather than limit the invention.

EXAMPLE 1 Thirty grams of safflower oil is placed in a glass beaker. The beaker is floated on a pool of mercury in a closeable reaction vessel. An electrical connection is made to the pool or mercury so that it may function as a ground electrode. A doughnut shaped high tension electrode consisting of quartz vessel filled with mercury is lowered into the beaker so that it is 4 mm. above the surface of the oil. The reaction vessel is closed and purged with hydrogen. A static hydrogen atmosphere is maintained at a pressure of 1.08 atmospheres. The beaker forms one dielectric barrier of inch thickness and the quartz vessel forms a second dielectric :barrier of A inch thickness. An alternating current having a frequency of 10,000 c.p.s. is connected to the ground electrode and to the high tension electrode. A voltage ranging from 10.1 to 11.8 kv. peak is impressed across the electrodes for a period of 80 minutes. A corona power of 7 watts is employed. A temperature of 50 C. is maintained within the reaction vessel. A corona discharge is generated in contact with the upper surface of the oil. The safiiower oil is visually inspected before and after corona treatment. The results are set out in Table I. The hydrogen atmosphere is examined by mass spectrometry and found free of ester cleavage products. Acid number tests of the ester indicate no decarboxylation.

EXAMPLE 2 The process of Example 1 is repeated substituting oleic acid for safllower oil, except that a voltage of 11.2 to 13.9 kv. peak is employed for a period of 60 minutes. The oleic acid is visually inspected before and after corona treatment. The results are set out in Table I. Freedom from cleavage products and decarboxylation is determined as in the case of safilower oil.

EXAMPLE 3 TABLE I Energy input Appearance (watt- Material hr./gm.) Before After Saflflower oil- 0. 21 Oil, faintly ye11ow- Oolciirtliess oil, white so Oleie acid 0. 22 Yellow oil Slushy solid. Linoleie acid---. 0.49 ..do Do.

EXAMPLE 4 A beaker containing 50 grams of methyl linoleate is placed in a corona reactor of the type shown in FIG. 1 of the commonly assigned application of Browne et al., Ser. No. 409,199, filed Nov. 5, 1964. The reactor is equipped with two electrodes, the upper of which is provided with a inch quartz barrier. The bottom of the beaker provides a second ,11 quartz barrier adjacent the bare lower electrode. The reactor is first purged with argon and then a flow of 4.6 cc./min. hydrogen is circulated through the reactor at slightly above atmospheric pressure. The reactor is operated at a frequency of 10,000 c.p.s. and voltages of approximately 20 kv. peak. The upper electrode is maintained A inch above the upper surface of the ester. The methyl linoleate is maintained at approximately 50 C. during corona treatment.

The methyl linoleate is subjected to a corona discharge propagated in the hydrogen atmosphere. After a treatment of 1 watt-hour/ gram, a 10 gram sample is removed with a pipette and labeled Sample A. The remainder is again treated with a corona discharge until a total treatment of 2 watt-hrs./gra.m is attained. Twenty grams of the remainder is labeled Sample B. The finally remaining twenty grams of methyl linoleate is further treated with a corona discharge until a total treatment of 4 watthours/ gram is attained. The finally treated twenty grams is labeled Sample C.

Samples A, B, and C as well as an untreated sample of methyl linoleate are subjected to a molecular weight determination using a vapor pressure osmometer and benzene as a solvent. The accuracy of the test is corroborated by the fact that the molecular weight of untreated methyl linoleate is found to be 294, the theoretical value. The weight percentage acid dimer in the treated methyl linoleate samples is determined by the formula Weight percent acid dimer= M1) where M =molecular weight of monomer, M =molecular weight of dimer, and M=expe1imentally determined average molecular weight The test results are set out in Table II.

Five grams samples, two of oleic acid and three of linoleic acid, are successively treated in a corona reactor of the type described in Example 4. The reactor in each instance is first purged with argon and then a flow of 4.6 cc./ min. hydrogen is circulated through the reactor at slightly above atmospheric pressure. The upper electrode is maintained inch above the upper surface of the sample, and the sample is maintained at approximately 50 C. during corona treatment. A corona frequency of 10,000 c.p.s. is employed. The voltage, wattage, energy input, and dimerization are indicated in Table III. Acid number tests of each sample indicated no decarboxylation.

EXAMPLE 10 The procedure of Examples 5-9 inclusive is repeated except that a 100 gram sample of stearic acid is used. Since stearic acid is normally a solid at room temperature, the acid is heated to approximately 70 C. initially preceding corona treatment. Sufficient heat is generated in the corona treatment to maintain the acid in the liquid form. The test results are set out in Table III.

TABLE III Voltage, Energy input Dimerization k.v.p. (watt-hrs./grn.) (wt-percent) 22. 0 2 l3. 8 22. 1 0. 4 l7. 0 20. 0 0. 1 l2. 1 20. 0 0. 4 15. 0 20. 0 1. 12 21. 2 l3. 5 0. 8 4. 9

EXAMPLE 11 A gram sample of linoleic acid is circulated at a rate of 315 grams/minute through a swirl tube reactor of the type disclosed in FIG. 1 of the commonly assigned application of Dibelius et al., Ser. No. 411,192, filed Nov. 16, 1964, now US. Pat. No. 3,342,721, issued Sept. 19, 1967. Hydrogen is circulated through the reactor at slightly greater than atmospheric pressure and at a rate 5.3 cc./min. No exterior electrode cooling is employed, however the center electrode is water cooled to maintain a reactor temperature of 43 C.

A corona discharge is propagated by operating the reactor at approximately 12 kv. peak, a frequency of 10,000 c.p.s., and a corona power of 281 watts. The corona treatment is continued for 60 minutes at the end of which period the linoleic acid is noted to be considerably increased in viscosity. The total energy input is 2.34 watthrs./gram while the power density, based on the area of the inner electrode, is 6.5 watts/ink EXAMPLE 12 a voltage of approximately 1-6 kv. peak is applied at a reactor temperature of 51 C. The corona is propagated 7 for 33 minutes. During the latter portion of the run, a gel forms on the inner wall of the reactor. The treated linoleic acid is a semisolid at 25 C. An energy input of 2.06 watt-hrs./ gram is supplied.

EXAMPLE 14 The swirl employed in Example 11 is plugged below the corona generating zone with glass wool. The corona generating zone is filled with 8 to 12 mesh alumina. A 120 gram sample of linoleic acid is circulated through the reactor at a rate 18.5 grams/minute while hydrogen is circulated through the reactor at a rate of 4.8 cc./minute. The inner electrode is water cooled so that it is maintained at a temperature of 47 C.

The corona reactor is operated at a voltage of approximately 19 kv. peak and a frequency of 10,000 c.p.s. The power density, based on the area of the inner electrode, is 5.6 watts/m At the end of 38 minutes, with an energy input of 1.24 watt-hrs./ gram, the linoleic acid gels within the packed bed.

EXAMPLE 15 A 350 ml. petri dish is provided with 100 g. of linoleic acid and placed in a corona reactor of the type employed in Example 4. Hydrogen is circulated through the reactor at a rate of 850 cc./ min. A corona discharge is generated within the hydrogen atmosphere at a voltage of 28 to 31.9 kv. peak and 10,000 c.p.s. The temperature of the sample is maintained at approximately 50 C. during corona treatment. An energy input of 1.14 Watt-hrs./ gram is supplied to the linoleic acid at a power input ranging from 7.8 to 13.6 watts. The linoleic acid remained a liquid although some solidification was noted around the edge of the petri dish. Also some solidified material was noted to have diffused onto the upper electrode. The material exhibited an iodine number of 92.1 after treatment as opposed to an initial iodine number of 119.7. The molecular weight increased from an initial 280 to 886.

While we have described our invention in terms of certain illustrative examples, it is apparent that numerous modifications will be readily suggested to those skilled in the art. For this reason, it is intended that the scope of the invention be determined with reference to the following claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1 A process of preparing unsaturated dimer acids comprising:

placing in a reducing atmosphere consisting essentially of hydrogen maintained at a pressure in excess of one atmosphere a material chosen from the group consisting of a monocarboxylic acid having from 0 to 3 ethylenic linkages and from 2 to 30 carbon atoms, and

an ester of said monocarboxylic acid and an alcohol having from 1 to 6 carbon atoms, and

subjecting the material to a corona discharge produced by an alterating current at a frequency of at least 3000 cycles per second in the amount of 0.01 to 10 watt-hrs. gram.

2. A process according to claim 1 in which the material is subjected to 0.1 to 5 watt-hrs./ gram.

3. A process according to claim 1 in which the reducing atmosphere is continuously circulated through the corona discharge.

4. A process according to claim 1 in which the material is continuously circulated through the corona discharge.

5. A process according to claim 1 in which the corona discharge is propagated in a packed bed of particulate dielectric material.

References Cited UNITED STATES PATENTS 3,166,537 1/1965 Gregg et al. 204157.1 3,304,249 2/1967 Katz 204164 3,404,078 10/ 1968 Goldberger 204164 OTHER REFERENCES Ellis, Hydrogenation of Organic Substituents, D. Van Nostrand, 3rd ed. 1930, p. 618.

Manual of Electrochemistry of Japan (Denki-Kaeaku Kyo Kai) 1954, pp. 1051-2.

E. Eichwald, Zeitschrift fiir Angewandte Chemie, 1922, vol. 35, No. 74, pp. 505-6.

ROBERT K. MIHALEK, Primary Examiner U.S. Cl. XrR. 260537 

